There is now a sound experimental basis for believing that human breast skin temperature may provide an ovulation predictor which will indicate the timing of ovulation approximately two to four days prior to ovulation and three to five days prior to the post-ovulatory rise in basal body temperature. The evidence for such an hypothesis has been gathered over a period of four years in a series of pilot studies in the Principal Investigator's laboratory at the University of California and (while on sabbatical leave) at the Institute of Animal Physiology, Babraham, England. This evidence has led to the suggestion that circulatory changes occur in the human breast which lead to an increase in breast temperature in response to an increase in circulating estrogen, and consequently that the pre-ovulatory estrogen surge results in a transient increase in breast temperature which can provide a prospective indication of ovulation. If this phenomenon is found to be a general and reliable one, it could provide a simple and readily acceptable method of ovulation prediction highly useful in fertility management. The present proposal is for a four-year intensive study of twenty-four subjects for three months each, using radiotelemetry, infra-red thermography and laboratory measurement of thermal and circulatory variables, to ascertain the characteristics, consistency and underlying physiology of the pre-ovulatory breast temperature peak, together with related animal experiments studying details of the mechanisms generating this peak.